The principal aim of the proposed research is to study the effect of ionizing radiation on the frequency of insertion and excision of transposable elements in Drosophila. The possible synergistic effect of X-rays on transposon mobility will be measured in dysgenic hybrid female Drosophila by the induction of X-linked MMS-sensitive insertion mutants, defective in DNA repair. The distribution of DNA insertion mutants will be determined for the eleven known X-linked DNA repair loci by in situ hybridization to labeled transposable element probes and the specific locus mutation confirmed by complementation analysis. The effect of ionizing radiation on reversion rate of the isolated insertion mutants will also be measured in dysgenic females and the frequency of insertion and excision at DNA repair loci will be compared to that at the singed locus, a known hotspot of insertion mutagenesis. Recent developments have implicated chromosomal rearrangements rather than point mutations as the genetic basis for the oncogenic process. This is exemplified in the case of retroviruses whose genomic organization is very similar to that of the Drosophila transposable elements. Thus it is important to relate the effects of ionizing radiation, an established oncogenic agent, to the known molecular effects involved in carcinogenesis. The use of the Drosophila transposon model system appears to be a reasonable approach to accomplish this aim. A secondary goal of the proposed research is to use one or more isolated insertion mutants as a means of cloning DNA repair genes. Cloning of these genes can provide important insight into gene sequence organization, gene copy number as well as the relationship between specific structural changes in the gene and the functional defect. Since a link between DNA repair and cancer has been established, the elucidation of the genomic organization, function and regulation of a DNA repair gene(s) in a genetically well-defined eukaryote is essential.